ams: mark ams::Result [[nodiscard]] (partially complete).

NOTE: This work is not yet fully complete; kernel is done, but
it was taking an exceedingly long time to get through libstratosphere.
Thus, I've temporarily added -Wno-error=unused-result for libstratosphere/stratosphere.

All warnings should be fixed to do the same thing Nintendo does as relevant, but this
is taking a phenomenally long time and is not actually the most important work to do,
so it can be put off for some time to prioritize other tasks for 21.0.0 support.

fusee/program/source/fusee_setup_horizon.cpp

@@ -300,8 +300,9 @@ namespace ams::nxboot {
                     /* If we should, save the current warmboot firmware. */
                     UpdateWarmbootPath(expected_fuses);
                     if (!IsFileExist(warmboot_path)) {
-                        fs::CreateDirectory("warmboot_mariko");
-                        fs::CreateFile(warmboot_path, warmboot_src_size);
+                        /* Try to create the directory/file, allowing them to fail (if already exist). */
+                        static_cast<void>(fs::CreateDirectory("sdmc:/warmboot_mariko"));
+                        static_cast<void>(fs::CreateFile(warmboot_path, warmboot_src_size));
 
                         Result result;
                         fs::FileHandle file;